MICROBIAL MINING IN ACID-SULFATE SPRINGS: INDICATORS OF EPITHERMAL ORE DEPOSIT FORMATION

In circum-neutral pH hot springs, siliceous sinters, Fe-oxides represent the dominant mineralogy. The formation of these minerals in hot springs is often microbially induced. In addition, microorganisms in hydrothermal systems commonly develop biofilms in order to locally control T, pH, salinity, and oxygen saturation, which may also sequester toxic metals and concentrate limiting nutrients needed for growth. Microbial metabolic processes commonly results in the sequestration trace metals including Ag, Au, As, and Sb within the sediment oxide fraction. The purpose of this study was to examine the impact of microorganisms on metal partitioning in an acid-sulfate hydrothermal system to determine the role microorganisms play in trace metal sequestration.

We characterized the influence of microbes on trace metal adsorption in two acid-sulfate hot springs on the southwest flank of Rincon de la Vieja, Costa Rica. 16S rRNA analyses of microorganisms from within Pailas de Agua I, reveal several species of algae, including Galderia sp., Cyanidium sp, a number of g-proteobacteria , Acidithiobacillus caldus, and Euryarcheota, including a number of uncultivated microorganisms and methanogens.

Water temperatures vary from 79.6 -89.3 ^oC, and are characterized by low pH (2.6-4), high ionic strengths (I= 0.47-0.79), high concentrations of Fe (4-6 mM), SO₄^2- (15-38 mM). Trace metal concentrations, are below detection limits in solution. Sediment samples were analyzed via X-ray diffraction (XRD) and sequential extraction (SE). XRD analysis indicated spring sediments were primarily composed of Fe/Al clay minerals, 2- and 6-line ferrihydrite, goethite, and hematite, quartz, and opal-a.

Sequential extraction showed that Fe and S which partition into the oxide and exchangeable fractions, respectively. With the exception of gold, SE shows that trace metals, including Cu, Ni, As, and Ag preferentially partition into the organic fraction. Transmission electron microscopy shows Cu sorption directly to microbial cells and biofilm. The concentrations of these metals, with the exception of Cu are present within the organic fraction in excess of average crustal values and calculated microbial toxicity concentrations. We posit microorganisms sequester trace metals to reduce metal toxicity in high T, acid environments.